1. Field of the Invention
This invention relates to amorphous metal magnetic components; and more particularly, to a generally three-dimensional bulk amorphous metal magnetic component for large electronic devices such as magnetic resonance imaging systems, television and video systems, and electron and ion beam systems.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) has become an important, non-invasive diagnostic tool in modern medicine. An MRI system typically comprises a magnetic field generating device. A number of such field generating devices employ either permanent magnets or electromagnets as a source of magnetomotive force. Frequently the field generating device further comprises a pair of magnetic pole faces defining a gap with the volume to be imaged contained within this gap.
U.S. Pat. No. 4,672,346 teaches a pole face having a solid structure and comprising a plate-like mass formed from a magnetic material such as carbon steel. U.S. Pat. No. 4,818,966 teaches that the magnetic flux generated from the pole pieces of a magnetic field generating device can be concentrated in the gap therebetween by making the peripheral portion of the pole pieces from laminated magnetic plates. U.S. Pat. No. 4,827,235 discloses a pole piece having large saturation magnetization, soft magnetism, and a specific resistance of 20 xcexcxcexa9-cm or more. Soft magnetic materials including permalloy, silicon steel, amorphous magnetic alloy, ferrite, and magnetic composite material are taught for use therein.
U.S. Pat. No. 5,124,651 teaches a nuclear magnetic resonance scanner with a primary field magnet assembly. The assembly includes ferromagnetic upper and lower pole pieces. Each pole piece comprises a plurality of narrow, elongated ferromagnetic rods aligned with their long axes parallel to the polar direction of the respective pole piece. The rods are preferably made of a magnetically permeable alloy such as 1008 steel, soft iron, or the like. The rods are transversely electrically separated from one another by an electrically non-conductive medium, limiting eddy current generation in the plane of the faces of the poles of the field assembly. U.S. Pat. No. 5,283,544, issued Feb. 1, 1994, to Sakurai et al. discloses a magnetic field generating device used for MRI. The devices include a pair of magnetic pole pieces which may comprise a plurality of block-shaped magnetic pole piece members formed by laminating a plurality of non-oriented silicon steel sheets.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved pole pieces. This is so because it is these pieces which are essential for improving the imaging capability and quality of MRI systems.
Although amorphous metals offer superior magnetic performance when compared to non-oriented electrical steels, they have long been considered unsuitable for use in bulk magnetic components such as the tiles of poleface magnets for MRI systems due to certain physical properties of amorphous metal and the corresponding fabricating limitations. For example, amorphous metals are thinner and harder than non-oriented silicon steel and consequently cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating bulk amorphous metal magnetic components using such techniques commercially impractical. The thinness of amorphous metals also translates into an increased number of laminations in the assembled components, further increasing the total cost of the amorphous metal magnetic component.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material making it very difficult to cut or form easily, and once annealed to achieve peak magnetic properties, becomes very brittle. This makes it difficult and expensive to use conventional approaches to construct a bulk amorphous metal magnetic component. The brittleness of amorphous metal may also cause concern for the durability of the bulk magnetic component in an application such as an MRI system.
Another problem with bulk amorphous metal magnetic components is that the magnetic permeability of amorphous metal material is reduced when it is subjected to physical stresses. This reduced permeability may be considerable depending upon the intensity of the stresses on the amorphous metal material. As a bulk amorphous metal magnetic component is subjected to stresses, the efficiency at which the core directs or focuses magnetic flux is reduced. This results in higher magnetic losses, increased heat production, and reduced power. Such stress sensitivity, due to the magnetostrictive nature of the amorphous metal, may be caused by stresses resulting from magnetic forces during operation of the device, mechanical stresses resulting from mechanical clamping or otherwise fixing the bulk amorphous metal magnetic components in place, or internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material.
The present invention provides a low-loss, bulk amorphous metal magnetic component having the shape of a polyhedron and being comprised of a plurality of layers of ferromagnetic, amorphous metal strips. Also provided by the present invention is a method for making a bulk amorphous metal magnetic component. The magnetic component is operable at frequencies ranging from about 50 Hz to 20,000 Hz and exhibits improved performance characteristics when compared to silicon-steel magnetic components operated over the same frequency range. More specifically, a magnetic component constructed in accordance with the present invention and excited at an excitation frequency xe2x80x9cfxe2x80x9d to a peak induction level xe2x80x9cBmaxxe2x80x9d will have a core loss at room temperature less than xe2x80x9cLxe2x80x9d wherein L is given by the formula L=0.0074 f (Bmax)1.3+0.000282 f1.5 (Bmax)2.4, the core loss, the excitation frequency and the peak induction level being measured in watts per kilogram, hertz, and teslas, respectively. Preferably, the magnetic component will have (i) a core-loss of less than or approximately equal to 1 watt-per-kilogram of amorphous metal material when operated at a frequency of approximately 60 Hz and at a flux density of approximately 1.4 Tesla (T); (ii) a core-loss of less than or approximately equal to 12 watts-per-kilogram of amorphous metal material when operated at a frequency of approximately 1000 Hz and at a flux density of approximately 1.0 T, or (iii) a core-loss of less than or approximately equal to 70 watt-per-kilogram of amorphous metal material when operated at a frequency of approximately 20,000 Hz and at a flux density of approximately 0.30T.
In a first embodiment of the present invention, a bulk amorphous metal magnetic component comprises a plurality of substantially similarly shaped layers of amorphous metal strips laminated together to form a polyhedrally shaped part.
The present invention also provides a method of constructing a bulk amorphous metal magnetic component. In a first embodiment of the method, amorphous metal strip material is cut to form a plurality of cut ferromagnetic amorphous metal strips having a predetermined length. The cut strips are stacked to form a bar of stacked ferromagnetic, amorphous metal strip material and annealed to enhance the magnetic properties of the material. The annealed, stacked bar is impregnated with an epoxy resin and cured. The preferred ferromagnetic amorphous metal material has a composition defined essentially by the formula Fe80B11Si9.
In a second embodiment of the method, ferromagnetic amorphous metal strip material is wound about a mandrel to form a generally rectangular core having generally radiused corners. The generally rectangular core is then annealed to enhance the magnetic properties of the material. The core is then impregnated with epoxy resin and cured. The short sides of the rectangular core are then cut to form two magnetic components having a predetermined three-dimensional geometry that is the approximate size and shape of said short sides of said generally rectangular core. The radiused corners are removed from the long sides of the generally rectangular core and the long sides of the generally rectangular core are cut to form a plurality of polyhedrally shaped magnetic components having the predetermined three-dimensional geometry. The preferred amorphous metal material has a composition defined essentially by the formula Fe80B11Si9.
The present invention is also directed to a bulk amorphous metal component constructed in accordance with the above-described methods.
Bulk amorphous metal magnetic components constructed in accordance with the present invention are especially suited for amorphous metal tiles for poleface magnets in high performance MRI systems; television and video systems; and electron and ion beam systems. The advantages afforded by the present invention include simplified manufacturing, reduced manufacturing time, reduced stresses (e.g., magnetostrictive) encountered during construction of bulk amorphous metal components, and optimized performance of the finished amorphous metal magnetic component.